Internal Server Error

I’ve been building my new WordPress Adventure Journal site over the past week and it’s been working great. But all of a sudden now when I make a quick change to the menu and try to save it, I get this message:

The server encountered an internal error or misconfiguration and was unable to complete your request. Please contact the server administrator and inform them of the time the error occurred, and anything you might have done that may have caused the error. More information about this error may be available in the server error log.

I can save and publish posts and pages just fine; it’s just updating the menu that gets this error. Can anyone tell me what it means and how to fix it?